A stand-alone loudspeaker system can include one or more transducers in a cabinet or box. The cabinet is often constructed using a heavy duty, high-density material to provide a rigid or stiff cabinet structure that isolates a transducer's front-propagating acoustic wave signal from its rear-propagating wave signal. Among other benefits, a rigid cabinet can resist rear-wave transmission of low frequency energy or vibration, and can help to prevent second-order low frequency signal propagation that can degrade system performance. In addition, a rigid or stiff cabinet can improve a low frequency transducer's effectiveness by helping to focus the transducer's energy into a listening space.
As cabinet rigidity or stiffness is decreased, more low frequency energy can be lost in excitation or pumping of the cabinet. Such excitation of a cabinet can result in the cabinet itself acting as a transducer, leading to signal degradation and fidelity loss, such as due to second-order low frequency signal propagation, or due to phase cancellation. When a cabinet is a stand-alone box, the cabinet generally has limited influence on other environmental structures or elements, such as a floor upon which the cabinet sits. Typically, a floor is made of a relatively strong, stiff material, thus minimizing transmission of any vibrations from a loudspeaker to the mechanical structure of the surrounding environment.
Flush-mounted, in-wall or in-ceiling loudspeaker systems present different performance challenges and design compromises as compared to stand-alone cabinet-type loudspeaker systems. For example, due to size or other constraints, some flush-mounted in-wall or in-ceiling loudspeakers may not include a sufficient rear-wave containment structure to prevent unwanted signal propagation or to help focus the loudspeaker's energy.
In-wall or in-ceiling loudspeaker systems can be mounted to surfaces that are fabricated from lightweight materials, such as drywall. In comparison to materials used in stand-alone cabinet-type loudspeaker systems, such wall and ceiling materials can be prone to mechanical excitement. Thus, vibration or sound, such as generated by a loudspeaker that is mounted to a wall or ceiling in one portion of a building structure, can be unintentionally transmitted into other areas of the building structure, such as into adjoining rooms.
Some loudspeaker systems include multiple transducers. For example, a loudspeaker system can include a low frequency transducer and a coaxial high frequency transducer. To help compensate for a low frequency transducer's inherently poor mid frequency off-axis response or directivity, some systems that include a relatively large low frequency transducer (e.g., 8″ or greater) can include mid and high frequency transducers to improve the system's off-axis frequency response or directivity. In some examples, multiple high frequency transducers can be used, and each of the high frequency transducers can be responsible for reproducing a specified frequency band.
Deploying multiple high frequency transducers in close proximity to one another can disrupt the transducers' intrinsically wide and smooth off-axis response. However, positioning the multiple transducers together in an array, such as along a single line, can minimize disruption to the horizontal off-axis frequency response. The frequency response along the transducer's vertical axis of alignment may be diminished, however, due to the close proximity of the transducers. As a result, such systems can exhibit relatively wide horizontal directivity and relatively narrow vertical directivity. The heightened vertical directivity tends to diminish reflections on the floor and ceiling of a room and is thought to enhance speech intelligibility.